1. Preliminary Considerations

The problem of the mind–body relation is deemed to be a consequence of mind–body dualism.Footnote ² The related problem of mind–body union has been insightfully captured by Hume: ‘there is no question in philosophy more abstruse than that concerning […] the nature of the uniting principle, which constitutes a person’.Footnote ³ The difficulty of both problems is undeniable. Where appropriate I shall draw upon some of the vast literature, while not losing sight of my primary engagement with the depth of Descartes’ arguments.

There are Scholastic, Augustinian, and classical antecedents to Descartes’ philosophical concerns. I shall not follow this path – not because I think he wrote in a vacuum, but because my aim is to attend carefully to Descartes’ notions and arguments; as he says: ‘I wish to point out here that I am paying no attention to the way these terms have lately been used in the Schools […] when my own views are profoundly different’.Footnote ⁴ For example, Descartes’ use (in his various Replies to Objections, the Principles, the Correspondence, and elsewhere) of what is translated as ‘soul’ is to be understood neither in a theological sense as spirit, nor in Aristotle's sense as anima, but as mind (mens): ‘I use the term ‘mind’ [mens] rather than ‘soul’ [anima], since the word ‘anima’ is ambiguous and is often applied to something corporeal [as in Aristotle's use of psyche]’.Footnote ⁵ ‘Anima in good Latin signifies air, or breath; it is in a transferred sense, I think, that it means mind. That is why I said that it is “often taken for a corporeal thing”’.Footnote ⁶ Aristotle too distinguishes between psyche and nous: thinking mind or intellect.Footnote ⁷

The mind–body relation and the mind–body union hold between the mind and the human body, not body.Footnote ⁸ ‘Body’, strictly, refers to ‘the whole universe of corporeal substance [with] no limits to its extension’.Footnote ⁹ Descartes’ substance dualism is between mind and body; the human body, strictly, is not a substance.Footnote ¹⁰ He uses the ordinary phrase, prevalent at the time, ‘res sive substantia’, thing or substance, to refer to individual bodies, such as the human body,Footnote ¹¹ but strictly they are modes of the one corporeal substance under its principal attribute of extension.Footnote ¹² The mind is a substance whose principal attribute is thought; willing, understanding, imagining, perceiving, sensing are faculties, not separable parts, of it – they presuppose the same I and the unity of self-consciousness.Footnote ¹³ Its modes are its acts, ideas, mental states, and are indivisible.

What is divisible is ‘one piece of matter’.Footnote ¹⁴ Substances, corporeal or thinking, and their respective principal attributes, are indivisible: unity and inseparability are constitutive of what it is to be a substance. Divisibility means capable of being divided at least ‘in my thought’,Footnote ¹⁵ is not for Descartes a mereological thesis, but a commitment to the continuous nature of a body (he also holds a non-mereological thesis for time and space). For ease of expression, I shall sometimes use ‘mind–body’, but I shall mean the mind and the human body.

2. Causal Principle

Causality is central to Descartes’ metaphysics and to his conception of the mind–body relation. It's thus necessary to elucidate as clearly as possible his Causal Principle, not only in order to overcome objections that he didn't argue for the heterogeneous interaction of mind and the human body,Footnote ¹⁶ but also because any elucidation of his Principle involves a host of other key notions also requiring a clear elucidation: efficient and formal causes; degrees of reality or perfection; Axiom IV: formally or eminently. In an attempt to overcome numerous misconceptions, the notion of ‘degrees of reality or perfection’ and Axiom IV will be discussed under separate sections; however, as will be evident, they're not independent of but integral to Descartes’ Causal Principle.Footnote ¹⁷

In the Third Meditation Descartes presents the Causal Principle thus: ‘it is manifest by the natural light that there must be at least as much <reality> in the efficient and total cause as in the effect of that cause’.Footnote ¹⁸ First, the Causal Principle makes no assumptions that there are in fact causes, and therefore begs no questions; the proposition ex nihilo nihil fit – nothing comes from nothing – is not presupposed by the Causal Principle but follows from it.Footnote ¹⁹ Rather, the natural light of reason – which is not an arbitrary assumption but a precondition of self-consciousness – clearly manifests what the relation must be between cause and effect if there are causes or effects: the effect (i) can be equal to but not greater than, (ii) is dependent on, and (iii) cannot be prior, in the ontological order, to the cause.Footnote ²⁰

Secondly, the Causal Principle makes no assumptions, a priori or empirical, about the nature of the relata.Footnote ²¹ Causality is a metaphysically basic category, along with such basic common notions as substance, essence, unity, truth; basic common notions are attributable indifferently to mental and corporeal entities. Causality can be given an elucidation, but it's unanalysable and irreducible. Notions such as force, contact, impact, transfer of energy, interaction, or power presuppose causality; they're causal notions, and thus cannot ‘provide a basis for an account of causality’, contrary to what some philosophers argue,Footnote ²² nor can ‘causality just [be] a manifestation of these powers’.Footnote ²³ Causality is itself neither mechanical nor non-mechanical, neither physical nor mental; the notions of mechanical, non-mechanical, physical, or mental interactions make appeal to causality. We can identify ways of causality's use, but it itself remains primary to such accounts. The neutrality of the Principle bridges the metaphysical distinctness of the two substances.

Thirdly, Descartes accepts formal cause, the cause of essenceFootnote ²⁴ – in esse – following Aristotle's to ti en einai – the what it is to be – ‘taking the whole essence of a thing to be its formal cause’Footnote ²⁵ as distinct from efficient cause, the cause of coming into being – in fieri. Although ‘the two kinds of cause are different’,Footnote ²⁶ both are central to his metaphysics and are constitutive of the Causal Principle, which refers to ‘the efficient and total cause’, and both are required for any total explanation, irrespective of the relata.

Fourthly, the Causal Principle is not committed to the ‘causal-likeness principle’ in terms of efficient cause, but only of formal cause, ‘the cause of being [or essence] itself’,Footnote ²⁷ which is the infinite entity. Attributions of the causal-likeness principle to Descartes, including rejections of it, fail to draw his distinction.Footnote ²⁸ Principles IV.198, however, seems to suggest otherwise: ‘we understand very well how the different size, shape and motion of the particles of one body can produce various local motions in another body. But there is no way of understanding how these same [properties] (size, shape and motion) can produce something else whose nature is quite different from their own – like the substantial forms and real qualities which many <philosophers> suppose to inhere in things’.

Responses to this passage are unconvincingFootnote ²⁹ because the contentious part is left untouched: ‘whose nature is quite different’. Descartes is not only arguing that substantial forms are incomprehensible, but that they're of a different nature – they're non-beings, or ‘only chimeras’,Footnote ³⁰ consisting merely in words not in reality, and without efficacy. Principles IV.198 continues: ‘we cannot understand how these […] forms could have the power subsequently to produce local motions in other bodies’. It's thus inconceivable that any real entity can causally interact with non-beings, and vice versa, whereas mind and the body are both real entities, possessing a degree of power and independence (see sections 3–4 below).

Fifthly, Descartes – followed by Spinoza and Leibniz – also understands cause as a rational notion in terms of intelligibility, reasons, and normativity, as in causa sive ratio,Footnote ³¹ sometimes referred to as the Principle of Sufficient Reason.

My elucidation of Descartes’ Causal Principle and defence of its neutrality regarding the nature of relata, demonstrate that his replies to various critics are well-grounded. For example, he replies to Gassendi's Counter-Objections regarding the mind–body relation: ‘the whole problem contained in such questions arises simply from a supposition that is false and cannot in any way be proved, namely that, if the soul and the body are two substances whose nature is different, this prevents them from being able to act on each other’.Footnote ³²

Gassendi and other commentators, past and present, assume, without independent arguments, that causation is a physical relation and hence heterogeneous interaction puts pressure on the intelligibility of mind–body interaction, or is inconsistent with Descartes’ Causal Principle.Footnote ³³ As his robust response to Gassendi shows, Descartes didn't think there was a problem, and didn't require homogeneity between the relata, as I have demonstrated.

There's also no indication in his Causal Principle that Descartes restricts efficient causes to rational agents, as Garber thinks;Footnote ³⁴ or that all Cartesian causes are formal, as Hatfield,Footnote ³⁵ Nadler,Footnote ³⁶ and others argue.Footnote ³⁷ On the contrary, Descartes extends efficient causes to material things, such as the fire causing the wax to meltFootnote ³⁸ or wood to burn.Footnote ³⁹ He strips powers of substantial forms and defends substance activity, arguing that minds and bodies are causally efficacious without substantial forms.Footnote ⁴⁰ His commitments are clear: ‘[when] I attributed to the body no power of self-movement […] these were simply commonly held views which I was rehearsing so as to show in the appropriate place that they were false’.Footnote ⁴¹

3. Degrees of Reality or Perfection

The natural light of reason manifests that ‘there must be at least as much <reality> in the efficient and total cause as in the effect of that cause’. What does Descartes mean by ‘degrees of reality or perfection’ (realitatis sive perfectionis)? Is it intractable, mysterious, barely comprehensible as has been argued by a number of commentators (including Hobbes, AT VII.185)? First, in this context, ‘perfection’ denotes completeness – from perfectionem Footnote ⁴² – such that an infinite entity would be the supremely complete or perfect being, whereas a finite entity is complete or perfect of its kind.

Secondly, ‘reality’ here denotes formal, intrinsic, reality or being – the ‘degree of reality or being’Footnote ⁴³ that a true entity possesses in itself – not to be confused with existence. Reality or being is itself eternal whether it pertains to a finite or an infinite entity and admits of degrees; existence neither admits of degrees, nor is it eternal. Except with the concept of God, only the possibility of existence is ‘contained in the concept […] of everything that we clearly and distinctly understand’.Footnote ⁴⁴ In Descartes’ metaphysics, being or what is real is what is true, and what is true is being or real – ‘truth consists in being’Footnote ⁴⁵ – and has a degree of power and independence.

Degrees of reality or perfection denote degrees of power and independence. There's an unmistakable reciprocal link between completeness and true unity, being, and power. A degree of power and independence pertains to the nature of what it is to be a substance, corporeal or thinking. Power and independence can be elucidated thus:

1. (A) Power – attributed indifferently to finite corporeal or thinking substance as much, or in so far, as it is in itself,Footnote ⁴⁶ to sustain itself, to act, to affect, to bring about an effect, to move, to interact, to assent, to suspend, and so on. The proviso signifies a true or real entity, capturing a finite substance's degrees of power; the proviso can also mean naturaliter, suggesting that what a real entity does, it does naturally by its own power, or from its own nature, or in virtue of its own being.Footnote ⁴⁷ The mind and the human body have the power to act on each other.

2. (B) Independence can be understood in three ways:

   1. (i) Ontologically – in terms of prior/posterior, independence/dependence. Principles I.51, offers a clear exposition of ontological independence/dependence: ‘By substance we can understand nothing other than a thing which exists in such a way as to depend on no other thing for its existence. And there is only one substance which can be understood to depend on no other thing whatsoever, namely God’ – the infinite substance. ‘In the case of all other substances [finite], we perceive that they can exist only with the help of God's concurrence. Hence the term ‘substance’ does not apply univocally […] to God and to other [substances]’.

      Let's elucidate the metaphysical modalities: only the infinite substance is in itself simpliciter explanatorily and ontologically, i.e., does exist independently. A finite substance, in so far as it is in itself, can exist independently; the modal ‘can’ captures a substance's degrees of independence (unlike an attribute that can exist only in substance – ‘in’ denotes dependence). It's surprising how many commentators claim that in Descartes’ system, finite substances do exist independently contrary to Principles I.51–52, and Fourth Set of Replies AT VII.226: ‘the notion of a [finite] substance is just this – that it can exist by itself’. Thus in terms of ontological independence: ‘Two or more substances are said to be really distinct when each of them can exist apart from the other’;Footnote ⁴⁸ they can be ontologically independent, not that they are.

   2. (ii) Metaphysically – in terms of metaphysical independence (or distinctness) of nature or essence. Except in God, there is a distinction between essence, what it is, and existence , that it is. The mind, whose principal attribute is thought ‘is of a nature entirely independent of […] body’,Footnote ⁴⁹ and body, whose principal attribute is extension, is of a nature entirely independent of mind. Metaphysical independence or distinctness of nature or essence implies ontological separability not separation (as Def.X makes clear). Metaphysically distinct kinds of substance is what Descartes understands by ‘The Real Distinction between Mind and Body’ – by Substance Dualism: mind and body are really distinct and can exist without each other.Footnote ⁵⁰ The argument is not that they do exist independently of, or separately from each other. Put differently, real distinction or independence of nature or essence, does not mean actual ontological separation, only separability, a logical/ontological possibility.Footnote ⁵¹

   3. (iii) Explanatorily – the nature or essence of mind, in so far as it's simply a thinking thing, is clearly and distinctly understood independently of, or apart from, a clear and distinct understanding of the nature or essence of body, in so far as it is simply an extended thing, and vice versa.Footnote ⁵² Explanatory independence of clear and distinct understanding of nature or essence of both entities, implies ontological separability not separation.

In all three ways, independence implies existential/ontological separability not separation (except for God). Attributes (which constitute the essence of substance) and modes (by which a finite substance principal attribute is made known in a determinate way) are neither ontologically nor metaphysically nor explanatorily independent of the substance to which they pertain.

My discussion and elucidation of ‘degrees of reality or perfection’ draw support, not from commentators but only from Descartes’ works: for example, from his reference to his understanding of the infinite supremely real or perfect being as ‘independent […and] supremely powerful’,Footnote ⁵³ or of ‘the immense […] power that is [presented] within the idea of God’.Footnote ⁵⁴ The infinite being has supreme formal reality, power and independence; it's prior ontologically, metaphysically, and explanatorily.

Given that degrees of reality or perfection denote degrees of power and independence, it follows that the more reality or perfection something has, the more power and independence it has.Footnote ⁵⁵ We're therefore not speaking of beauty and other properties, as Spinoza points out in his Descartes’ Principles of Philosophy,Footnote ⁵⁶ stressing that reality doesn't mean properties, but only ‘formal reality or being’. Yet commentatorsFootnote ⁵⁷ interpret ‘degrees of reality or perfection’ to mean ‘properties’, and are consequently led astray, referring to the Causal Principle as a containment principle, claiming that the cause contains properties which it passes on or transfers to the effect.

4. Axiom IV: Formally or Eminently

Descartes argues that the reality of the effect must be in the cause either formally or eminently.Footnote ⁵⁸ The ‘in’ denotes an ontological and explanatory independence/dependence relation,Footnote ⁵⁹ not a spatial or containment relation. The effect depends for its reality, its power, on the cause. It's reasonable to understand this to mean that by virtue of coming into being, the effect must have a degree of reality or power (e.g., in accordance with some law – such as the third law of motionFootnote ⁶⁰ – governing the interaction among things).

The reality is in the cause formally if the effect has as much reality – as much power – as the cause; for example, a child's reality or perfection is as much as that of the parents because they are all true substantial unities of equal ontological status. This is so, even though the parents are ontologically prior in existence to the child, but not vice versa.Footnote ⁶¹ If there's more reality, more power in the cause than in the effect, the reality is in the cause eminently, that is, to a higher degree: an architect is of a higher degree of reality than her drawing, which is an artefact. Both the child and the drawing ‘can remain in existence quite apart from the “cause” in this sense’,Footnote ⁶² because we're concerned with coming into being (in fieri) or efficient cause, not with ‘total cause, the cause of being [in esse] itself’ or formal cause which is the infinite being.

Commentators who think that ‘reality or perfection’ means property are led to claim that Axiom IV is difficult ‘to make completely clear’ to ‘the point of intractability’, or that ‘eminently’ means having ‘some grander property’,Footnote ⁶³ or ‘more excellent’ properties,Footnote ⁶⁴ and so on.Footnote ⁶⁵ O'Neill claims that according to Axiom IV ‘the properties of extended substance may be contained eminently in the mind’.Footnote ⁶⁶ This is puzzling, not least because it's impossible for corporeal properties to be contained eminently (or otherwise) in a mind. In the passage on which O'Neill draws, Descartes is concerned with the ideas of corporeal properties, not the properties themselves. Thus when he supposes ‘it seems possible that [the ideas of these modes] are contained in me eminently,’ he means, because I am a substance with greater formal reality, it's possible to be the cause of the objective reality of the ideas of corporeal modes.Footnote ⁶⁷ Having misconstrued Axiom IV as ‘passing on of properties’ and as a ‘formal containment principle’,Footnote ⁶⁸ and conflated it with the notions of formal and objective reality of ideas, it's a short step to claiming that it's intractable or ‘not possible to decide what Descartes’ considered view was […] whether there are two causal principles or one’.Footnote ⁶⁹

Gassendi (despite thinking he's offering an objection to Descartes) captured the crucial point that degrees of reality and Axiom IV have nothing to do with properties, when he says: it doesn't follow ‘that a father, in begetting his son, chops off a part of his rational soul and gives it to him’.Footnote ⁷⁰

Pulling together my three-part discussion of the key notions integral to Descartes’ metaphysics of causality, demonstrates a cogent and clear understanding of his single Causal Principle. The reality, truth, and power that pertain to the nature of mind and the body can be expressed thus: the mind possesses the power to think, will, act, perceive, feel, interact with, and be affected by the body, and the body possesses the power to move, to be affected by and interact with the mind and with other bodies.

5. Substantial Union of the Mind and the Human Body

Having drawn the Real Distinction between the essence of mind and the essence of body – in the Sixth Meditation AT VII.78, which implies separability not separation – Descartes turns to consider what he calls the unio substantiale between the mind and the human body. The self appeals to what his nature, which arises from the substantial union, teaches him: ‘when I feel pain there is something wrong with the body […]. So I should not doubt that there is some truth in this’,Footnote ⁷¹ that I am united with my body.

What philosophical reasons does Descartes have for defending this statement? In his Counter-Objections Gassendi raised, among others, the question: ‘how can the soul move the body if it is in no way material?’ Descartes replied: ‘These questions presuppose amongst other things an explanation of the union between the soul and the body’.Footnote ⁷²

Descartes’ defence is found in the Sixth Meditation, in his letters to Regius, to Arnauld, to Princess Elizabeth, in which he refers to two simple notions, extension and thought, and to the simple common notion of union;Footnote ⁷³ the simple common notions are attributed indifferently to corporeal and to thinking things.Footnote ⁷⁴

The simple notions and the simple common notions were elucidated in the Regulae some twenty years earlier, and later in the Meditations and the Principles.Footnote ⁷⁵ ‘To this class [of simple common notions] we must also refer those common notions which are, as it were, links which connect other simple natures together, and whose self-evidence is the basis for all the rational inferences we make’.Footnote ⁷⁶ The simple common notion of union falls in this class; it's not psychological, or non-philosophical, nor was it surreptitiously introduced in his letter to Elizabeth, in which he clearly treats all three notions (soul, body, union) equally as ‘three kinds of primitive ideas or notions, each of which is known in its own proper manner’.Footnote ⁷⁷ Simple and common notions are the simplest constituents of knowledge, and are known through themselves.Footnote ⁷⁸ They're unanalysable, primitive, irreducible and prior in the order of reasoning.

In defending the substantial union, Descartes writes to Regius: ‘a human being is a true ens per se [a true entity in itself], and not an ens per accidens [accidental entity]’. The mind and the human body are united not by ‘the mere presence or proximity of one to another, but by a true substantial union’.Footnote ⁷⁹ What Descartes is expressing here is the fact that the mind–body union cannot in any way, metaphysical, epistemic, or scientific be the same as, or compared to other objects whose interaction presupposes no union. It follows that the efficient causal bidirectional interactions of mind–body can be neither remote nor proximate, but arise from the union as Descartes rightly argues. This is so, even if causal interactions between other objects and the human body, or between a person and other objects, can be characterised as remote or proximate. And it is so, in various passages where Descartes says, for example, the ‘mind is immediately affected […] by the brain’.Footnote ⁸⁰ ‘Immediate’ is not equivalent to ‘proximate’; it means ‘directly, or without mediation’, as opposed to ‘next to’, ‘very close in space (or time)’, or ‘immediately preceding or following in a series’.

As I shall argue in section 8, a substantial union is presupposed only by consciousness and self-consciousness – by what it is to be a human being, or a sentient being. A further metaphysical tenet, as we saw earlier and which will be important when we turn to the mind–body relation, is the reciprocal link between true unity and being, and activity and being.

His advice to Regius is that he could explain the union, as he – Descartes – did in his Metaphysics ‘by saying that we perceive that sensations such as pain are not pure thoughts of a mind distinct from a body, but confused perceptions of a mind really united to a body’.Footnote ⁸¹ ‘Confused’ from its Latin etymology ‘confusus’ from ‘confundere’ means mixed or mingled – the mixing of reason and the senses. ‘Confused sensory perceptions’ or ‘confused modes of thinking’ doesn't mean bewildered, perplexed, or as Alanen says ‘most […] confusing thoughts,’ involving ‘inherent confusion,’ or are ‘inherently confused’.Footnote ⁸² It means mixed not distinct because they ‘arise from the union’.Footnote ⁸³ There is no inconsistency in saying that a sensory perception is clear and confused, since the latter is contrasted with distinctness; clarity is contrasted with obscurity. However, whatever is distinct must also be clear and must be true.

Sensations, emotions, passions, etc., fall ‘under the common concept of […] consciousness, and we call the substance in which they inhere a thinking thing or a mind’.Footnote ⁸⁴ They're not a third category since they cannot be without ‘an intellectual substance to inhere in […] because there is an intellectual act included in their essential definition’.Footnote ⁸⁵ That is, they're modes of the mental substance under its principal attribute of thought: the distinction ‘between them and myself’ is modal. An intellectual act is a manifestation of self-consciousness. Self-consciousness implies the capacity for self-attribution of those modes, the capacity for self-reflection, the ability to step back from them, a step towards a more objective standpoint, and to subject them to self-governed critical evaluation and scrutiny.

Nevertheless, the fact that I am, say, in pain presupposes the mind–body union; that is, sensations, emotions, etc., fall also under the general faculty of sentience or sensory awareness, not only under the general faculty of intellectual awareness as cogitations. They presuppose the union, they ‘arise from the union’, they don't constitute it; the person is directly aware of them because of a true substantial union.

Descartes presents and defends his metaphysics of the unity of self-consciousness, a necessary condition for integrating the senses, imagination, reason, and the will; the unity of self-consciousness presupposes an irreducible I, without which no reasoning, no inference, no judgement, no complete thought, no knowledge, no experience would be possible.Footnote ⁸⁶ The mind–body union epitomises his metaphysics of personhood, to which the self is not an appendage but the source of a unifying notion of personhood: a person is a self-conscious rational, free, acting human being that has reason and reflection and can think of itself as itself – an autonomous individual with epistemic and moral responsibility.

6. The Metaphysics of the Union

The metaphysics of the union or personhood can be given a two–fold defence: first, ‘the union […] is not accidental to a human being, but essential, since a human being [a person] without it is not a human being’:Footnote ⁸⁷ it's an irreducible unity per se,Footnote ⁸⁸ whose true nature arises from the union. Secondly, ‘the human body has all the dispositions required to [be united with a] soul’, and without these dispositions it's not ‘strictly a human body’,Footnote ⁸⁹ including ‘in itself all the dispositions required to preserve that union’.Footnote ⁹⁰ If the body has these dispositions ‘then short of a miracle it must be united to a soul’.Footnote ⁹¹

It's evident that for Descartes, a person is neither a construct or a neurobiological particular, nor a disembodied mind or ego (despite misattributions), as is shown from his denial of Arnauld's suggestion that a person is simply a mind that happens to have a body as a vehicle,Footnote ⁹² and his affirmation that a person is an irreducible, unanalysable true entity – neither analysable nor reducible to either a mind or a body.Footnote ⁹³

The mind–body union can, in turn, be given a two–fold elucidation and defence. Epistemically or phenomenologically, sensations are a gateway to my awareness that ‘I am not merely present in my body as a sailor is present in a ship but I am very closely joined [conjunctum]’ to it. If this were not so – if, for example, the body were damaged, yet I felt no pain – I should simply ‘have an explicit understanding of the fact’.Footnote ⁹⁴

Metaphysically, sensations are not only subjective conditions (or awareness) that I am substantially united; this the critics of the union may concede, but ask: how am I so united? Metaphysically, sensations also presuppose the a priori notion of union which has objective validity; the union, or ‘the uniting principle, which constitutes a person’, is founded in reality, it's not up to us. This shows how the epistemic or phenomenal awareness – what the subject experiences – and the metaphysical requirement of union are connected.Footnote ⁹⁵ Implicit in this defence is an answer to: ‘why is the union a presupposition of the interaction?’ – to which I return below.

When we reach the Sixth Meditation, whose commitments are metaphysical, based in ‘an order corresponding to the actual truth of the matter’, the unity of self-consciousness extends one's self-knowledge: the self's knowledge of itself as a free, active, real being existing as an embodied being. The mind and the human body are united in such a way that they act upon each other.

7. The Metaphysics of the Mind–Body Relation

The discussion of the mind–body relation spans this and the following three sections. I begin with the question: what is the nature of the mind–body relation? It's not parallelism, occasionalism, pre-established harmony, isomorphism, or supervenience. It's a bidirectional causal interaction – it arises naturally from the substantial union, the relation doesn't constitute the union. Central to the metaphysics of mind–body relation is the Causal Principle and its neutrality regarding the nature of the relata.

Princess Elizabeth and Gassendi, along with past and present critics of Descartes’ metaphysics, insist on the homogeneity of the relata of causal relations. Elizabeth's most pressing questions centre on the thesis: ‘it seems that all determination of movement happens through the impulsion of the thing moved, by the manner in which it is pushed by that which moves it’.Footnote ⁹⁶ A similar objection is raised by Gassendi: ‘you still have to explain how that ‘joining and […] intermingling’ […] can apply to you if you are incorporeal, unextended and indivisible’.Footnote ⁹⁷ Elizabeth and Gassendi have no problem with physical causal relations in Descartes’ physics, but find the metaphysics of mind–body relation and of union problematic.

Descartes’ replies to both are very similar; to Elizabeth he writes: on the simple common notion of union ‘depends our notion of the soul's power to move the body, and the body's power to act on the soul and cause its sensations and passions’.Footnote ⁹⁸ He goes on: ‘I think that we have hitherto confused the notion of the soul's power to act on the body with the power one body has to act on another’.Footnote ⁹⁹ To Gassendi he replies: ‘when you try to compare the intermingling of mind and [the] body with the intermingling of two bodies, it is enough for me to reply that we should not set up any comparison between such things because they are quite different in kind’,Footnote ¹⁰⁰ though they are both subject to the single Causal Principle.

8. The Heart of my Argument: the Uniqueness of the Union

They are indeed quite different in kind; but why is it enough for Descartes to reply as he does? Why is the substantial union a presupposition of the mind–body relation? When the problem is raised in this way, it becomes clear that there's an underlying thesis that needs to be brought out. It's enough for Descartes to reply as he does because there's a fact of the matter regarding the difference in kind: what is explicitly unique and marks that ‘difference in kind’ is a metaphysical necessity, the presuppositionality, of the substantial union, without which a human being would not be a human being – ‘a true mode of union, as everyone agrees, though nobody explains what this amounts to’.Footnote ¹⁰¹

What it amounts to, and what nobody has explained – then or now – because it has escaped them, is not only the truth of the substantial union, but its uniqueness. It's unique because innumerable causal interactions – body–body interactions – occur in the world without a presupposition of a union, without presupposing a principle of unity among them.

Mind-body causal interactions are different in kind, as Descartes rightly stresses. Any explanations of body–body causal interactions cannot therefore be used either against or in defence of mind–body and body–mind causal interactions, because these interactions could not occur without the mind-body substantial union. The reason for this constraint, and the answer to the question above, is that without the substantial union we would not have experiences, sensations, pains, feelings, or sense perceptions. Nor would the mind ‘incline its will’ to action.Footnote ¹⁰²

Without the presupposition and uniqueness of the substantial union, we would be detachedly aware of causal effects, like pilots in ships,Footnote ¹⁰³ having simply ‘an explicit understanding of the facts’ – but that's all contradicted by the irreducible and undeniable facts of self-conscious awareness. The substantial union is the only way to understand how we are, and why we feel so intimately bound up with our bodies.

Metaphysically, however, the substantial union does not follow from our experience or first-person awareness of interaction: it's presupposed by the interaction, a presupposition that can be clearly grasped by the intellect. If there is sentience or consciousness, and not only self-consciousness, there must be a substantial union. There is, it seems to me, a clear parallel between the metaphysics of the substantial union, and the metaphysics of the unity of consciousness.

Descartes, insightfully, turns past and present theses of mind-body interactions on their head: there is no more profound unity than that of mind-body union presupposed by mind-body interactions. Whenever and in whatever case the mind is involved, there must be a unity; in the case of rational free active beings, all the mind's acts presuppose the principle of unity of self-consciousness which presupposes an irreducible I or self. Metaphysically, the substantial union is the only way to understand what we are as persons: embodied rational agents who take epistemic and moral responsibility for our acts and actions.

This is consistent with there being intra–bodily relations as they occur in various functions of the human body, such as digestion, blood circulation, etc. It's consistent with the metaphysical possibility of pure thoughts, in so far as I am simply a thinking being.Footnote ¹⁰⁴ And it's consistent with there being intra–mental relations, since the mind has the power to produce thoughts as they occur in thinking. Descartes explains: ‘when [the cognitive power of the mind] acts on its own [without addressing itself to the imagination, or applying itself together with the imagination, to the senses], it is said to understand [clearly and distinctly]’.Footnote ¹⁰⁵ This activity pertains to the mind's own power: the mind ‘enjoys some liberty to think of other things than those presented by the senses’.Footnote ¹⁰⁶

With regard to mind–body causal interaction, we can appeal both to the Causal Principle – whose neutrality bridges the metaphysical distinctness of the two substances – and to the Principle that powers are constitutive of what it is to be a substance. The neutrality of the Causal Principle doesn't trivialise it; on the contrary, it's a substantive principle imposing genuine constraints on what the relation must be between cause and effect (see section 2 above). The two Principles together explain the bidirectional interaction between mind and the human body, arising naturally from the substantial union. They thus provide a basis for further investigation, epistemic or scientific. How all this will come about is for a future project to fill in, since the necessity of a metaphysical basis has now been demonstrated.

For my present task, it's reasonable to conclude: first, without the substantial union there would be no sensory experience at all, either in self-conscious beings or in conscious creatures. Without it there would only be ordinary pilot/ship causal relations. Secondly, and equally importantly, without the substantial union there would be no experiencing, say, pain as pain, or pleasure as pleasure, which manifests the capacity for self-ascription – a distinctive capacity of a self-conscious concept-using being, transforming the nature of the experience itself.

Descartes’ metaphysical enquiries and the strict order of reasoning led him to locate the mind–body relation in the fundamental naturally real substantial union (‘in’ denotes ontological dependence, not a containment relation) rather than appealing to supernatural accounts: in so far as the mind and the body are united, the movements in the brain, acting directly upon the soul, ‘are ordained by nature to make it have such sensations’.Footnote ¹⁰⁷ On the account and defence offered in this paper, it seems that, unless one has a different idea of what the metaphysics of the union is, which would require explaining what ‘the nature of the uniting principle is, which constitutes a person’ as an ens per se, it makes no sense to continue asking how the mind and the human body are united. The notion of union is prior ontologically to their interaction, it's prior in the order of reasoning, and unanalysably simple. Both in philosophy and in science, asking for an explanation of a basic notion would be asking for a more fundamental notion, and there isn't one; it's an ill-formed question. Otherwise, an infinite regress looms large.

9. Descartes’ Challenge and Ad Hominem Argument

In his letter to Elizabeth (21 May 1643), in the Sixth Set of Replies, and in his letter for Arnauld (29 July 1648), Descartes offers a challenge to their view by pointing to a contradiction between their objections to the mind–body relation and their understanding of the quality of heaviness (gravitatem) or gravity:

Descartes rejects Scholastic explanations in terms of substantial forms because they present heaviness as what ‘carried bodies towards the centre of the earth as if it had some knowledge of the centre within itself. […But] there can be no knowledge [or intentionality] except in a mind’.Footnote ¹⁰⁹ He doesn't suppose ‘there are in nature any real qualities, which are attached to substances, like so many little souls to their bodies’,Footnote ¹¹⁰ as if a body tended of its own volition and agency towards a particular location. An attempt to project mental activity onto the material world would be unacceptable and a misuse of the notion of mental activity. He thus rejects the idea that body–body interaction can be understood through understanding mind–body interaction, not only because our free, intentional acts can no more clarify body-body interaction than the latter can clarify the former, but also because substantial forms are non-beings, ‘are not real’Footnote ¹¹¹ and, equally importantly, because body–body interaction presupposes no substantial union.

This approach is consistent with seeing Descartes as offering an ad hominem argumentFootnote ¹¹² – though not in any sense of attacking Elizabeth, especially given his next letter: ‘I did not worry about the fact that the analogy with heaviness was lame because such qualities are not real’.Footnote ¹¹³ That is, he's not arguing that because the true explanation of heaviness doesn't involve contact, impact, etc., therefore there's no problem with seeing mind–body interaction in a similar way. Rather, he's pointing to a contradiction in his critics’ view: as they're happy to accept an explanation of heaviness that doesn't involve impact, nor do they insist that ‘this motion is produced by a real contact between two surfaces’ – in fact they ‘have no difficulty in conceiving how [heaviness] moves [a] body or how it is joined to it’ – why should they have any difficulty with seeing that mind–body interaction doesn't involve contact either?

This interpretation of his argument is consistent with Descartes’ own rejection of the theory of heaviness in question. Indeed, it turns the discussion found in the literature on its head, and rebuts attributions of inconsistency, namely: Descartes’ answer to Elizabeth is ‘inconsistent with the foundations Descartes gives to his theory of motion [in his physics]’.Footnote ¹¹⁴ Descartes’ answer to Elizabeth is correct and consistent (see also below). Such objections can be traced to a lack of understanding of the uniqueness of the substantial union, and to a misplaced demand for an explanation of one primary notion in terms of another.Footnote ¹¹⁵ ‘That the mind, which is incorporeal can set the body in motion is […] one of those self-evident things which we only make obscure when we try to explain them in terms of other things’.Footnote ¹¹⁶ Moreover, such objections can be traced to a lack of understanding the metaphysical neutrality of Descartes’ Causal Principle.

10. Descartes’ Response to Elizabeth

Descartes writes to Elizabeth that he thinks it was her devoted time to meditating “rather than thoughts requiring less attention that have made Your Highness find obscurity in the notion we have of the union of the mind and the body’”.Footnote ¹¹⁷ He explains: ‘the soul is conceived by the pure intellect; body […] can likewise be known by the intellect alone, but much better by the intellect aided by the imagination [in terms of shapes, alluding to the role of diagrams]; and finally what belongs to the union of the soul and the body is known only obscurely by the intellect alone or even by the intellect aided by the imagination, but it is known very clearly by the senses’.Footnote ¹¹⁸ He continues: ‘It does not seem to me that [the intellect] is capable of forming a very distinct conception of both the distinction between the soul and the body and their union; for to do this it is necessary to conceive them as a single thing and at the same time to conceive them as two things; and this is absurd’.Footnote ¹¹⁹ It's ‘the ordinary course of life […] that teaches us how to conceive the union of the soul and the body’.Footnote ¹²⁰

He offers a similar response to Arnauld regarding the mind–body interaction: ‘it is something which is shown to us […] by the surest and plainest everyday experience’. (29 July 1648 AT V.222: CSMK:358) Descartes seems clear about what it means to affirm ordinary life. In his understanding of the ‘surest and plainest everyday experience’ and of scientific inquiry there is what might be called an interplay between a priori reasoning and experiential evidence.

11. The Intellect Conceiving the Union Clearly and Distinctly

How acceptable is all this within Descartes’ philosophical quest? In particular, how acceptable is it within his new metaphysics that the intellect seems incapable of a distinct conception of the mind–body union, and hence known to be indubitably true? This is considered to be a limitation, an exception to, or inconsistent with Descartes’ metaphysics. It's true that in his quest for the possibility of new metaphysics, of establishing scientia, Descartes would not affirm anything less than what is clearly and distinctly conceived. For the purposes of exploring the emerging new physics and examining the question of whether the nature of reality can be intelligible to reason, Descartes was right about the appropriateness and rigour of his metaphysical inquiry and his deeper concerns.Footnote ¹²¹ The objections regarding a limit, an exception to, or inconsistency in his metaphysics seem therefore well placed.

Drawing on my two–fold defence of the mind–body union – the epistemic and metaphysical – I suggest that Descartes is entitled to more than we have understood him to claim.Footnote ¹²² It seems that without meditating but not without attentive thoughts – as he explains to Elizabeth – which require ‘some degree of rationality’Footnote ¹²³ freed from preconceived opinions, the intellect is capable of forming a clear and distinct conception of the union.

How is that possible? It's possible because the intellect can focus attentively on what is clear and distinct, a single thing, a unit,Footnote ¹²⁴ signified as a whole – without insisting that this ‘license the inference that [mind and the body] are one and the same thing’.Footnote ¹²⁵ Descartes explains that it doesn't seem to him that the intellect ‘is capable of forming a very distinct conception of both the distinction between the soul and the body and their union; for to do this it is necessary to conceive them as a single thing and at the same time to conceive them as two things; and this is absurd.’Footnote ¹²⁶ Given the undeniability of this logical point about simultaneous conceivability, he didn't think it needed spelling out: it's logically impossible clearly and distinctly to conceive a real distinction between two entities and at the same time to conceive them very distinctly as a single thing, a unit.

The two entities or ‘what belongs to the union of the soul and the body is known only obscurely by the intellect’ because of the intermingling of the intellect and the senses; and pre-philosophically, it's clearly known by the senses.Footnote ¹²⁷ But philosophically or metaphysically the intellect can ‘conceive them as a single thing’, a unit.Footnote ¹²⁸ That is, the intellect can attentively conceive the union ‘in its own proper manner’ as distinct from, and ‘not by comparison with any of the others’ – without, that is, at the same time conceiving the mind and the body as distinct.Footnote ¹²⁹ Simple and common notions are more akin to demonstratives than to descriptions: our attentive thoughts can focus on that notion as a notion in and of itself, signifying a whole. In fact, in Rule Twelve Descartes argues that the ‘self-evidence [of simple and common notions] is the basis of all the rational inferences we make’.Footnote ¹³⁰ They're ‘all self-evident and never contain any falsity’.Footnote ¹³¹

Therefore, the intellect can clearly and distinctly conceive the union because, I have argued, metaphysically the union presupposes the objective a priori notion of union – prior in the order of reasoning – whose self-evidence is the basis of the rational inferences we make in this area of metaphysics. When the intellect understands clearly and distinctly, it turns towards the notions ‘which are within it’,Footnote ¹³² the a priori self-evident notions of reason, not derived from sense–experience, but presupposed by it.

The arguments of this paper together constitute a defence of Descartes’ thesis that the mind–body interaction, presupposing their union, is ‘one of those self-evident things which we only make obscure when we try to explain them in terms of other things’.Footnote ¹³³ And given my defence that the intellect can clearly and distinctly conceive the union, it constitutes no exception to his new metaphysics.